The goal of this research is to understand the molecular mechanism by which 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) causes morphological and functional abnormalities in the developing zebrafish heart. Exposure of newly fertilized zebrafish embryos to less than 1 ppb TCDD causes decreased ventricular size, heart rate, and cardiac output as one of the earliest signs of toxicity in the developing zebrafish. These endpoints of toxicity are followed by decreased blood flow, edema, and circulatory collapse. The proposed research will take advantage of the well known advantages of the zebrafish as a model for vertebrate development to better characterize and understand this process. The three major aims of the proposal are as follows: 1) Determine how TCDD alters the processes of heart formation. We will identify changes in morphology, the temporal and spatial patterning of heart cell gene expression, transdifferentiation, and cell migration to form the structures that make up a functional heart. 2) Determine the site of action of TCDD within the developing zebrafish that leads to these changes. For these experiments we will specifically block or activate the AHR/ARNT pathway by expressing dominant negative and constitutively active AHR proteins from tissue-specific promoters in cardiomyocytes and in endocardial cells. 3) Identify the genes that are regulated in the heart in response to TCD activation of the AHR/ARNT heterodimer. This aim is based on the assumption that because the AHR/ARNT dimer forms an active transcription factor, at least some of the responses to TCDD will involve altered transcription of specific genes. We will use DNA microarrays to identify these genes, and morpholino oligonudeotide knockdown experiments to determine the importance of these genes in responses to TCDD and in normal heart development.